1. Field of the Invention
This invention relates generally to sensors, and, more particularly, to sensors of various analytes fabricated by positioning on a fiber field access block a corrugated measurand sensitive layer whose properties ar changed as a result of interactions with the environment.
2. Description of the Related Art
One conventional device for detecting the presence of analytes is an extrinsic polymer swelling sensor that uses reflections form a surface which is moved by the swelling of the polymer to provide an indication of the concentration of analytes. This type of sensor cannot be protected from the effects of temperature and is vulnerable to vibrations, pressure changes, and contaminants that may exist in the analyte.
Several other fiber optic chemical sensors rely on conventional spectroscopic instrumentation. In these cases the detector is usually a photomultiplier tube or a photodiode. The sources are usually a tungsten-halogen lamp, a xenon arc lamp, or an argon ion laser. These types of sensors require sophisticated, miniaturized demodulation equipment.
Other attempts have been made to use sensors to exploit properties of fiber optic media to transport light between a sample and the light source or detector. These techniques use light wavelengths in the blue or ultraviolet range in order to be sensitive to the spectral characteristics of the species formed by reaction of an indicator reagent with the analyte. These wavelengths require special lasers and are not transmitted well due to the absorption properties of the fibers. Additionally, the sensor stability is generally limited by the indicator. Since the detection mechanism requires photoexcitation, photodecomposition becomes an additional problem. Moreover, the associated dyes are usually unstable.
When light passes through a gradient in the index of refraction, such as may be created by an acoustical wave acting on the medium in which light is travelling, the light is diffracted. Structures with periodic distribution of the index of refraction are useful for providing phase matching between light modes that travel with different propagation velocities. This type of periodic distribution of the index of refraction is referred to as a grating structure.